The long-term goal of this project is to understand how solutes and water cross epithelia specialized for fluid transport, such as intestine, stomach, kidney, urinary bladder, and gallbladder. We propose to resolve barriers to nonelectrolyte permeation in epithelia, and to characterize channels for ion permeation, especially the amiloride-sensitive apical membrane channel of tight epithelia. To determine the relative resistances of the transjunctional and transcellular pathways to nonelectrolyte permeation, we shall compare the transepithelial flux of an nonelectrolyte with its separate effluxes from the loaded epithelium across the apical and basolateral cell membranes to the mucosal and serosal solutions. This method and three other methods will be used to resolve the relative resistances of the apical and basolateral membranes to non-electrolytes, and the nonelectrolyte permeability of the amiloride-sensitive pathway will be measured. Nitrogenous cations will be used as probes of this pathway in rabbit urinary bladder and toad urinary bladder. Noise analysis will be used to characterize this and other ion permeation channels in rabbit urinary bladder.